The present invention relates to cellular ceramic structures useful as filters for the casting of metal parts. The filters function to remove debris from a molten metal stream as it is cast into a mold, so that castings free of inclusions affecting the strength, homogeneity, performance, durability and/or appearance of the finished cast parts may be produced.
One type of molten metal filter for this use consists of a ceramic honeycomb body comprising a multiplicity of parallel through-channels through which a molten metal is to be cast. Examples of molten metal filters of this type are disclosed in U.S. Pat. Nos. 4,681,624, 5,281,462 and 5,104,540. In general, filters made in accordance with those patents are made by extrusion of a plasticized ceramic or mineral batch through a honeycomb die to form a green honeycomb shape, followed by firing of the green honeycomb to produce a strong, refractory channeled body having good filtration efficiency. As noted in U.S. Pat. No. 4,681,624, the honeycomb may in principle comprise any structure which has substantially parallel open channels, or longitudinally-extending cells, therethrough defined by relatively thin separator walls and in which the transverse cross-sectional configuration of the channels or cells is of any rounded (i.e., circular or elliptical) or polygonal (i.e., triangular, rectangular, hexagonal) shape. Ceramic compositions which may be used included zircon and zircon-containing compositions, as well as compositions of alumina, mullite, zirconia, spinel, cordierite, other aluminosilicates, aluminates, and titanates such as aluminum titanate.
At present, current extruded honeycomb molten metal filters of approximately 100 or 200 square-channel cell density, i.e., filters incorporating 100 or 200 square cpsi (cells per square inch) of filter frontal surface area, manufactured by honeycomb extrusion as described in the foregoing patents, are commercially employed. In general, these are considered have filtration efficiencies very similar to those of pressed ceramic filters of similar cell density. The latter filters are made from fired pressings of ceramic precursor materials rather than from extruded honeycombs of those materials, and they typically incorporate round rather than the conventional square channel or cell openings.
Although flow rates through the extruded square-cell honeycombs appear to be slightly higher than the pressed round-cell honeycombs under the same casting conditions (e.g., metal type, metal head, etc.), this faster flow characteristic is not a practical advantage at the casting rates currently prevalent in the industry. However, improvements in filter design which could provide better filtration, i.e., the ability to remove smaller debris from the molten metal stream, would be of substantial value in the industry, particularly if such an improvement could be obtained at similar cost and without any reductions in permissible casting rates.